This work was initiated to provide information about chemical structures as an aid in selecting compounds for screening from potential acquisitions. Statistics on structure-activity relations over broad classes of structures are being used to predict the probability of antitumor activity of new compounds in the mouse lymphocytic leukemia (P388) screen. In an attempt to uncover novel classes of active compounds, known classes of analogs have been removed from the training set. Also, a measure of rarity or uniqueness of a new structure is derived from the statistics on the structural characteristics of compounds that have been adequately screened in P388, regardless of their activity. The P388 model has been integrated into the DTP Chemical Information System for use in an experimental mode to determine whether it will aid in the selection of compounds for screening. This model will be updated as experiments using new chemical features demonstrate an improvement of performance.